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   难度：Medium
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   <h1 class="question_title">
    199. Binary Tree Right Side View
   </h1>
   <p>
    Given a binary tree, imagine yourself standing on the
    <em>
     right
    </em>
    side of it, return the values of the nodes you can see ordered from top to bottom.
   </p>
   <p>
    <strong>
     Example:
    </strong>
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   <pre>
<strong>Input:</strong>&nbsp;[1,2,3,null,5,null,4]
<strong>Output:</strong>&nbsp;[1, 3, 4]
<strong>Explanation:
</strong>
   1            &lt;---
 /   \
2     3         &lt;---
 \     \
  5     4       &lt;---
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   <h1 class="question_title">
    199. 二叉树的右视图
   </h1>
   <p>
    给定一棵二叉树，想象自己站在它的右侧，按照从顶部到底部的顺序，返回从右侧所能看到的节点值。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入:</strong>&nbsp;[1,2,3,null,5,null,4]
<strong>输出:</strong>&nbsp;[1, 3, 4]
<strong>解释:
</strong>
   1            &lt;---
 /   \
2     3         &lt;---
 \     \
  5     4       &lt;---
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